Why did the US drop the atomic bomb on Japan?

Why did the US drop the atomic bomb? President Harry Truman and his advisers used it on Hiroshima on 6 August 1945, and a second on Nagasaki three days later, to force Japan to surrender quickly and avoid an invasion that they expected to cost huge numbers of lives. Other motives, including impressing the Soviet Union, may have played a part. By the end of 1945 the two bombs had killed at least 140,000 people, and by some estimates more than 200,000, most of them civilians.

In short

• By summer 1945 Japan was losing but had not surrendered, and the US was preparing to invade it in November.
• Truman's government saw the bomb as a way to end the war fast; Japan surrendered on 15 August, after both bombs and the Soviet declaration of war.
• Historians still argue whether the bombs were necessary, and what most moved Japan's leaders to surrender.

Where did the war stand in summer 1945?

Germany had surrendered in May. Japan, which had brought the United States into the war with its attack on Pearl Harbor in 1941, had lost most of its navy and air force, and American bombers were burning its cities. A single firebombing raid on Tokyo on 9 and 10 March 1945 killed around 100,000 people. A US naval blockade was starving the country of fuel and food.

But Japan's leaders had not agreed to surrender. The battle for Okinawa, from April to June 1945, cost more than 12,000 American dead and far more Japanese soldiers and Okinawan civilians, and Japanese pilots flew suicide attacks, kamikaze, against American ships. To US planners, that looked like a preview of an invasion of Japan itself. Operation Downfall planned a landing on the southern island of Kyushu in November 1945 and on the main island near Tokyo in 1946.

How many lives an invasion would cost was, and is, disputed. The army chief of staff, George Marshall, in June 1945 expected losses in the first month on Kyushu similar to the battle for Luzon, about 31,000 American casualties. After the war, officials cited figures of hundreds of thousands to a million American casualties. The National Security Archive's collection of documents shows both, and historians argue whether the larger numbers were wartime estimates or later justification.

How was the decision made?

The bomb came out of the Manhattan Project, the secret US program to build a nuclear weapon, started in fear that Nazi Germany would build one first. The first test, at Alamogordo, New Mexico, on 16 July 1945, worked. Truman, who had become president only in April, was at the Potsdam conference with Stalin and Churchill.

From the start, US leaders had assumed the bomb would be used once it was ready. A committee of officials and scientists advised using it on a military-industrial target without warning. Some scientists, in the Franck Report, urged a demonstration on an uninhabited area instead; officials rejected that, fearing a dud or a shrug. Secretary of War Henry Stimson removed the ancient city of Kyoto from the target list. On 26 July the Allies issued the Potsdam Declaration, demanding Japan's unconditional surrender and warning of "prompt and utter destruction". It said nothing about the bomb, or about the emperor's fate. Japan's prime minister publicly dismissed it.

Date (1945)	Event
16 July	First atomic test in New Mexico
26 July	Potsdam Declaration demands unconditional surrender
6 August	Hiroshima: "Little Boy", a uranium bomb of about 15 kilotons, dropped at 8:15 a.m.
8 August	The Soviet Union declares war on Japan and invades Manchuria at midnight
9 August	Nagasaki: "Fat Man", a plutonium bomb, after clouds hid the first target, Kokura
10 August	Japan offers to surrender if the emperor keeps his position
14 August	The emperor breaks a deadlock in his cabinet and accepts the Allied terms
15 August	The emperor's recorded broadcast announces the surrender
2 September	Formal surrender aboard USS Missouri in Tokyo Bay

What happened in Hiroshima and Nagasaki?

Each bomb destroyed a city in an instant, with a blast, a flash of heat that burned people kilometers away, fire, and radiation. In Hiroshima, according to the Department of Energy's history, about 70,000 people died at once and the toll likely passed 100,000 by the end of 1945. The Atomic Heritage Foundation's estimates of deaths by the end of 1945 are about 140,000 in Hiroshima, a city of about 350,000, and about 74,000 in Nagasaki. Many of the dead were children, and they included thousands of Korean laborers and some Allied prisoners of war.

Survivors, the hibakusha, lived with burns, radiation sickness, higher cancer rates and the stigma that came with them. Their testimony became central to the movement against nuclear weapons, and in 2024 Nihon Hidankyo, their national organization, received the Nobel Peace Prize.

Why did Japan surrender?

Japan's leaders met in a deadlocked war council on 9 August. Three wanted to accept the Potsdam terms if the emperor's place was guaranteed; three, including the army minister, wanted further conditions and one last battle. The emperor's intervention broke the deadlock. In his broadcast on 15 August he cited a "new and most cruel bomb". The Soviet invasion of Manchuria, which ended Japan's hope that Moscow would help negotiate better terms, hit at the same moment. Which shock mattered more is the heart of the modern debate.

Were the bombs necessary?

Still argued

Historians have argued about this since the 1960s, and the positions are not simply for and against.

Position	The argument	Associated with
The bombs ended the war and saved lives	Japan's military would not surrender without a shock; the bombs prevented an invasion and months more of blockade and bombing that would have killed more people, Japanese included	Henry Stimson's 1947 defense; Richard Frank, Robert Maddox
They were not needed	Japan was near collapse; changing the surrender terms to keep the emperor, or waiting for the Soviet entry, could have ended the war; the bombs were also meant to impress the Soviet Union	Gar Alperovitz, Martin Sherwin
The Soviet entry mattered as much or more	The Soviet declaration of war destroyed Japan's last diplomatic hope; the bombs and the invasion together forced surrender	Tsuyoshi Hasegawa
The middle ground	The bombs probably shortened the war, but alternatives were not seriously tried, and US leaders treated using the bomb as a given	Barton Bernstein, J. Samuel Walker

The evidence shows that US leaders did see a diplomatic bonus in the bomb, as the State Department's historians note, and that Japan's leaders were divided until after both bombs and the Soviet attack. It cannot show what would have happened otherwise.

There is also a moral argument, separate from the military one: whether deliberately destroying cities full of civilians can be justified by lives it may save. Polls in the United States and Japan have long given very different answers.

What did the bombs change?

They ended World War 2, and they opened the nuclear age. The Soviet Union tested its own bomb in 1949, beginning the arms race of the Cold War. Nuclear weapons have not been used in war since Nagasaki. How this decision is remembered, in Hiroshima's Peace Memorial and in American museums, shows how history is argued over long after the facts are known.

Check yourself

• Without looking back: what did US leaders believe the alternative to using the bomb was, and how many lives did they think it would cost?
• Why do some historians say the Soviet declaration of war mattered as much as the bombs?
• What would have had to be different in July 1945 for Japan to surrender without the bombs, according to the critics?

Sources

• The atomic bomb and the end of World War II: a collection of primary sources — National Security Archive, George Washington University, ed. William Burr, 2020
• The atomic bombing of Hiroshima, 6 August 1945 — U.S. Department of Energy
• Survivors of Hiroshima and Nagasaki — Atomic Heritage Foundation, National Museum of Nuclear Science and History
• Potsdam Declaration, 26 July 1945 — National Diet Library of Japan
• Atomic diplomacy — Office of the Historian, U.S. Department of State